1. Field of the Invention
The present invention generally relates to image forming apparatuses, and more particularly to an image forming apparatus in which an optical scanning device and the like thereof are easily replaceable.
2. Background of the Related Art
As one example of the image forming apparatus, FIG. 1 shows a cross sectional view of a conventional laser printer such as that shown in Japanese Laid-Open Patent Application No. 60-95419. The laser printer generally comprises a photosensitive drum 1, a transfer charger 2, a charger unit 3, a discharge lamp 4, a developing part 5, a cleaning part 6, a toner cartridge 7, an optical scanning device 8, a paper supplying part 9, a fixing part 10, an ejecting paper switching part 11, a paper reversing part 12, an ejecting paper transport path part 13, paper supplying cassettes 14a and 14b, and paper trays 15a and 15b.
When a fault occurs during a paper ejecting operation, the ejecting paper transport path part 13 is opened as indicated by an arrow A so that the user may correct the fault. However, the conventional laser printer is designed with priority on the functions thereof over other features. In other words, when designing the laser printer, essentially no consideration is given to facilitate replacement of parts, removal of parts, and disassembling of parts. Because the design of the laser printer is function oriented, the construction and arrangement of the parts make it difficult to replace, remove and disassemble the parts of the laser printer.
For example, the quality of the recording made by the laser printer becomes poor when the optical scanning device 8 is not positioned correctly with respect to the photosensitive drum 1. For this reason, measures are taken so that the optical scanning device 8 is not easily removable.
FIG. 2 shows a perspective view of the optical scanning device 8 together with the photosensitive drum 1 which rotates in a sub scan direction. The optical scanning device 8 generally comprises an optical housing 800 which is mounted on a base plate 802. A polygonal mirror 803, a plurality of imaging lenses 804 arranged on an optical axis of a laser beam, a mirror 805 for detecting a horizontal synchronizing signal, a semiconductor laser 806, an optical connector 807 for making a connection with an optical fiber 807A which receives a reflected beam from the mirror 805, a lead connector 808 for the polygonal mirror 803 and a protective glass 809 for protecting the inside of the optical housing 800 from dust particles and the like. The optical connector 807, a connector of the semiconductor laser 806, the lead connector 808 of the polygonal mirror 803 and the like extend in various directions from side walls of the optical housing 800. Although not shown, a cover is mounted on the optical housing 800 by mounting screws for the purpose of preventing the laser beam from leaking to the outside and also to prevent dust particles and the like from entering the optical housing 800. To meet safety requirements, the cover cannot be removed by the user or the serviceman.
The optical housing 800 is mounted on the base plate 802 by at least four mounting screws 801. The optical housing 800 is fixed in position relative to the base plate 802 perpendicularly to an optical scanning plane (main scan direction A) of the laser beam. In order to remove the optical housing 800, it is necessary to pull the optical housing 800 in a direction B in FIG. 2, that is, in a direction along the height of the optical housing 800. For this reason, a space for accommodating the optical housing 800 in the laser printer needs to have a height greater than the height of the optical housing 800. But in the laser printer, parts such as the paper trays 15a and 15b exist above the optical housing 800 in the direction B, and these parts are fit from various directions making it virtually impossible to remove the optical housing 800 unless other parts in the way are first removed.
Generally, the laser printer has such a construction that the area occupied in the plan view is considerably large compared to the area occupied in the side view. In other words, the height of the laser printer is small compared to the width or depth thereof, and it is undesirable from the positional relationship of the optical housing 800 with respect to the neighboring parts to remove the optical housing 800 in the direction along the height thereof.
The laser printer described heretofore, a copying machine and the like which use the electrophotography method basically carry out the same process except for the respective optical systems. Each of these types of apparatuses require the supply of the toner, and use units which require replacement after the serviceable lives thereof end. Examples of such units are the photosensitive unit, the developing unit, the cleaning unit and the charger unit.
Some of these types of apparatuses not only enable the supply of the toner by the user, but also have user-replaceable photosensitive unit and cleaning unit. The demand for enabling the maintenance of the apparatus by the user is strong especially for the printer, and it is becoming important that various units are easily user-replaceable.
An example of the conventional apparatus of this type has a replaceable assembly integrally comprising the photosensitive unit, the developing unit, the cleaning unit and the like. When the serviceable life of one of the units ends, the whole replaceable assembly is replaced by a new replaceable assembly. However, the serviceable lives of the units of the replaceable assembly mutually differ because the wear of each unit depends on the characteristic conditions in which the apparatus is used. Hence, when the serviceable life of one unit ends, it is possible that the remaining units of the replaceable assembly can still be used for a relatively long time. But in this type of apparatus using the replaceable assembly, the whole replaceable assembly must be replaced by a new replaceable assembly when the serviceable life of one of the units ends even though the remaining units can still be used. Therefore, there is a problem in that the running cost of the apparatus is high since all of the units of the replaceable assembly are changed when the serviceable life of one of the units ends.
In order to eliminate the problem of the apparatus above described, there has been proposed an apparatus which enables the cleaning unit, the developer unit, the photosensitive unit and the like to each be independently replaced by a new unit when the serviceable lie thereof ends. In other words, only the unit with no more serviceable life is replaced by a new unit. There are generally two methods of making the units of the apparatus independently replaceable.
According to a first method, each unit is independently removed from the apparatus and replaced by a new unit. Hence, a mechanism is provided for each unit so that the unit can be easily pulled out by the user for the replacement. But each unit must be accurately positioned with respect to the other units, and from this point of view, it is undesirable to make each unit independently removable. In addition, each unit is accommodated within a limited small space in the apparatus, and it is extremely difficult for the user to work within the small space. Because of the small space in which the user must work to replace the unit, the unit being removed or the new unit being set may hit and damage other units which can still be used. In an extreme case, the toner may be spilled which would considerably damage the inside of the apparatus.
On the other hand, according to a second method, a pull-out module integrally comprises the units having mutually different serviceable lives. When one unit of the pull-out module needs to be replaced by a new unit, the entire pull-out module is pulled out from the apparatus, and only the unit having no more serviceable life is replaced by the new unit. Each unit is accurately positioned with respect to the other units in the pull-out module, and the positional relationship of the units is maintained when the pull-out module is pulled out from the apparatus. But because the units are accurately positioned within the pull-out module, it is difficult to remove the units from the pull-out module. Accordingly, there are problems in that the user may spill the toner, and damage the photosensitive unit and the like when replacing one unit of the pull-out module.
It is possible to make the units easily removable from the image forming apparatus, but in this case, the positioning of the units becomes unsatisfactory and the positional relationship of the units cannot be accurately maintained. The accurate positioning of the units related to the optical system of the image forming apparatus is essential in order to form an image of a satisfactory picture quality. On the other hand, then measures are taken to accurately maintain the positional relationship of the units, the removal of the units becomes extremely difficult as described before, and the possibility of damaging other parts of the image forming apparatus increases when replacing the unit. For these reasons, no image forming apparatus has been proposed with a replaceable optical unit (or module).
Further, the cover of the optical unit cannot be removed by the user or the serviceman to meet the safety requirements as described before. As a result, the optical unit cannot be repaired unless the entire image forming apparatus is taken to a repair station. From this point of view, it would be very useful if the optical unit were easily replaceable while maintaining the strict positional relationship with the other units of the image forming apparatus.